APP.] CHEMICAL BASIS OF THE ANIMAL BODY. 769 



time two compounds with this acid; these are soluble in water. It 

 possesses a pure blue colour; when pressed with a hard body a reddish 

 copper-coloured mark is left, and the crystals exhibit the same colour if 

 seen in reflected light. 



The soluble compounds with sulphuric acid give an absorption band 

 in the spectrum which lies close to the D line and to the red side of it. 

 This may be used to detect indigo. 



Treated with reducing agents, indigo is decolorised, being reduced to 

 indigo- white. The latter contains two atoms more hydrogen than indigo. 



Indol. C 8 H 7 N. 



To this body the specific odour of the faeces is partly due. It is 

 obtained as the final product of the reduction of indigo ; and also by the 

 distillation of proteid matter with caustic alkalis l . 



It often occurs among the products of the action of pancreatic 

 ferment on proteids ; its presence in such cases appears however to be 

 due, not to the action of the trypsin, but to a simultaneous putrefaction 

 under the influence of bacteria, etc.' If the pancreatic digestion be 

 carried on in the presence of salicylic acid, indol does not make its 

 appearance. Indol is a crystalline body, soluble in boiling water, 

 alcohol and sether. It passes over in the steam when its aqueous 

 solution is boiled. It is characterised by the following reactions. A 

 strip of pine-wood moistened with hydrochloric acid is coloured bright 

 crimson when dipped into a solution of indol. Its alcoholic solution 

 turns red when treated with nitrous acid and its aqueous solution gives 

 a copious red precipitate with the same reagent. It also yields a 

 characteristic crystalline compound with picric acid. 



Skatol. C 9 H 9 N (?). Noticed by Brieger 8 as one of the products of 

 putrefactive changes in the small intestine. Secretan 4 had previously 

 described a similar substance as arising from the putrefaction of 

 albumin. 



Skatol is crystalline and contains nitrogen; it is more soluble in 

 water than indol and does not give rise to any red colouration with 

 nitrous acid. 



Skatol readily passes into the urine when it occurs in the alimentary 

 canal, and then gives a violet-red reaction with strong hydrochloric acid. 



v. Nencki 5 prepares this substance by the putrefaction of a mixture 



1 Kiihne, Ber. d. deutsch. chem. Gesell. VHI. (1875), S. 206. 



2 Kiihne, Verhand. d. Heidlb. naturhist med. Ver. N.S. Bd. i. Hft. 3. Bericht 

 d. Deutschen chem. Gesellschaft, 1875, S. 206. 



3 Ber. d. Deutsch. chem. Gesell., Jahrg. x. (1877), S. 1027. 



4 Recherches sur putrefaction de Valbumine. Geneva, 1876. 



5 Centralb.f. d. med. Wiss., 1878, S. 849. 



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